Water is commonly injected downhole either as a means to improve oil recovery or as a means to dispose of waste water. Suspended solids in injected water tend to plug the formation and reduce the injectivity. Two plugging mechanisms occur--either the solids invade the porous media and clog pore throats, or the solids collect and build a filter cake.
Solids that invade the reservoir in-depth can cause permanent permeability damage because the solids are difficult to remove. On the other hand, if the solids form a filter cake within the well, the original permeability of the reservoir could be restored by removing the filter cake by back-flowing the well or by dissolving the filter cake with an acid solution. The nature of the solids and the reservoir dictate whether the plugging mechanism will be predominantly in-depth invasion or filter cake formation.
One common practice is to identify and eliminate processes that contribute to solids creation. For example, if a mix of incompatible waters causes solid scales to form, one would stop mixing these brines or would add a scale inhibitor to prevent solid formation.
Another common practice is to use surface filters to remove the suspended solids. Typically these filters are effective in removing the larger particles, leaving a minimal number of small particulates. But some operators are reluctant to install such filters on the surface facilities for such reasons as high capital and operating costs, problems in making the filters function correctly, and limited space on offshore platforms.